1. Field of the Invention
This invention concerns a cathode material for a lithium secondary battery and, more in particular, it relates to a cathode material for a lithium secondary battery in which an alkaline earth metal or the like is blended in a composite lithium nickelate-cobaltate, as well as a manufacturing method thereof.
2. Related Art
Lithium nickelate as a cathode material for lithium/ secondary batteries has a feature that the discharging capacity is large. However, when charge/discharge are repeated, lithium nickelate in which lithium ions are deintercalated tends to undergo phase change into a state with no lithium disorder. This trend is remarkable as the working temperature of the battery is higher.
Since the phase change is an irreversible reaction, the absolute amount of the lithium nickelate that contributes as the cathode material for the cathode active material results in lowering the discharging capacity. Further, evolved oxygen tends to react with the electrolyte constituting the battery and, accordingly, it may be even lead to a risk of ignition or bursting of the battery when the working temperature is high.
In order to solve the problem, Japanese Patent Laid-Open No. 23629/2001, for example, proposes a method of evaluating the thermal stability of the lithium secondary battery by using lithium nickelate as the cathode material to form a lithium secondary battery, conducting charging/discharging operation to the secondary battery thereby causing deintercalation of lithium ions from the lithium nickelate and then conducting thermogravimetry of the lithium nickelate. Further, as a preferred active material for use in the cathode of the lithium ion secondary battery evaluated by the method described above, it discloses a composition containing Co at a ratio of 0.05 to 0.3 mol based on one mol of Ni and further containing one or more of elements selected from B, Al, Mg, Ca, Sr, Ba, Fe, Ti, Zr, Y, La and Ce by 0.001 to 0.1 mol in total.
However, it has been found as a result of the study made by the present inventors that the safety of the battery can not be evaluated satisfactorily by the existing method described above. An actual example is shown below. It is considered that oxygen gas evolves at a high temperature region near 170° C. or higher on the cathode in a state where the battery is charged, and it causes concerns that the evolved oxygen and the electrolyte constituting the battery may react. In a case of evaluating the safety of the battery whose cathode material is exposed to a high temperature by measuring the decrease of the weight of the cathode material in a temperature change from 200 to 300° C. by thermogravimetry, although swelling or ignition of the battery can be estimated based on the decomposition gas from the cathode or the electrolyte, it can not be estimated that such an abnormality will occur in the battery by the starting of reaction between the oxygen evolving from the cathode and the electrolyte. As described above, the safety of the battery can not be recognized well by the existing method described above.